Chemical signal groups are widely used in chemical analysis to label substances of interest such as analytes, internal standards, comparison substances, and specific binding partners for analytes, so that such materials can be followed, detected, or determined in analytical procedures.
Examples of signal groups include radioactive atoms, fluorescent and luminescent molecules, metal-containing compounds, electron-absorbing groups, enzymes, and light-absorbing compounds.
Presently-used chemical signal groups suffer from a variety of shortcomings. Radioactive atoms in many cases have short half lives, present safety and disposal problems, and cause compounds containing them to be physically unstable and/or chemically labile. In addition, some radioactive materials do not provide high sensitivity, either because they do not produce a high level of radioactivity or because beta particles produced in the decomposition of the radioactive atoms are quenched by the medium to a substantial extent before they can be detected. Also, a variety of closely related radioactive tracers which might be employed and measured simultaneously in a single system are not available. Nonradioactive signal groups suffer from the deficiencies that the signal can be dependent on the environment of the label, which necessitates careful matching of the matrices of samples and standards if accurate data are to be obtained, that the effective signal can be reduced by any dilution of the sample during the analytical procedure, and that possibilities for using multiple labels simultaneously in a single analytical system are limited because of mutual interferences.
Traditional labels are typically retained on the labeled molecular species, and the presence or amount of the labeled material is determined by measuring the signal from the label while still attached to the remainder of the molecule, and often, in the presence of other constituents of the analytical system. As labeled species frequently contain a variety of moieties which can interfere with the measurement of the desired signal, and in addition, the labeled species cannot always be easily brought into a medium which is optimum for the measurement of the signal from the label, this can constitute a serious limitation on the utility of labels generally in a particular system, or on the use of particular labels which an investigator might wish to use.
An example of such traditional label usage is the common practice of labeling molecules with electron-absorbing groups. The molecules are inherently volatile or are rendered volatile by the labeling operation. They can then be determined in the gas phase by gas chromatography with electron capture detection (GC-ECD) or by GC with detection by electron capture negative ion mass spectrometry (GC-ECNI-MS).
The literature contains a few examples of indirect determinations of analytes by determination of a molecular species produced by decomposition of the analyte or chemical cleavage of a derivative of the analyte. An example of the former is the determination of trichloroacetic acid by decarboxylation and measurement of the resulting chloroform. See Buchet et al., Arch. Mal. Prof. Med. Trav. 35:395-402 (1974); and Senft, J. Chromatogr. 337:126-130 (1985). An example of the latter is the analysis for T.sub.4 toxin by formation of the labeled derivative N-(N-pentafluorobenzoyl-Met-Gly)-T.sub.4 followed by cyanogen bromide cleavage to produce N-pentaflurorobenzoyl homoserine lactone. See U.S. Pat. Nos. 4,650,750 and 4,709,016 by R. W. Giese.
Another example of an indirect determination of an analyte is shown in U.S. Pat. No. 4,629,689 of Diamond. This reference discloses analytical schemes in which, at the conclusion of a selective binding assay, an enzyme is present in a concentration and/or activity which is related to the amount of analyte present in the sample, and this enzyme is measured by measuring the amount of a readily detectable signal group released from a cleavable conjugate of the signal group and another molecular species by the action of the enzyme. As an example, the enzyme .beta.-galactosidase was determined by measuring the amount of o-nitrophenol released by the enzyme-catalyzed cleavage of o-nitrophenyl-.beta.-D-galactopyranoside.
It is very desirable to have labeling reagents which do not suffer from many or most of the above-described disadvantages of traditional reagents, and most importantly, permit multiple species to be labeled and determined in a single sample. Such reagents are the subject of the present application.